A hybrid vehicle that runs according to the driving force from at least one of the engine and motor is attracting attention as one measure directed to environmental problems. In such a hybrid vehicle, a high-voltage battery directed to running the vehicle, supplying power to the motor, is incorporated. The electricity stored in this running-purpose battery is also used for engine startup. Specifically, electricity is supplied to a motor generator coupled to the engine, and the engine is started by driving the motor generator serving as a motor.
In a hybrid vehicle, a subsidiary battery of low voltage for controlling the run as well as to supply electricity to accessories mounted on the vehicle is also incorporated. The subsidiary battery incorporated in a hybrid vehicle, as compared to those in a system that runs only by means of an engine, also functions as a power source for control of high-voltage systems including the running-purpose battery, in addition to the function as a power source for accessories. The importance of subsidiary batteries has become higher in accordance with the increase of the load. Conventional hybrid vehicles are mounted with a converter circuit that converts the electrical energy of the high-voltage system into low voltage to charge the subsidiary battery, taking into account the supply of electrical energy to the subsidiary battery (for example, refer to Japanese Patent Laying Open Nos. 2003-70103, 2003-189401, and 2001-320807).
In such hybrid vehicles, the running-purpose battery supplies electricity of a constant level to the accessories and subsidiary battery via the converter circuit, regardless of whether the vehicle is in an arrested mode or a running mode. In the case where the output performance of the running-purpose battery is extremely degraded due to being situated in a low temperature state, sufficient electricity from the running-purpose battery cannot be supplied to the motor generator when the engine is to be started by the motor generator. There was a problem that engine startability is not reliable.
To avoid such an event, Japanese Patent Laying-Open No. 2003-70103, for example, discloses a control apparatus for a hybrid vehicle characterized in that the drive of the converter circuit is stopped when the engine is to be started by means of the motor generator.
According to the control apparatus for a hybrid vehicle disclosed in Japanese Patent Laying-Open No. 2003-70103, control of automatic startup of the engine is effected by applying a driving force to the engine through the motor generator. When the capacity of the high-voltage battery is reduced at this stage, the electricity required to cause the motor generator to start the engine cannot be supplied from the high-voltage battery, leading to degradation in the startability. In view of this problem, the apparatus of Japanese Patent Laying-Open No. 2003-70103 stops the drive of the DC/DC converter when the engine is to be started by the motor generator to prevent the power output of the high-voltage battery from being drawn to the low-voltage battery. Thus, the engine startability is improved.
However, the prohibition of the drive of the DC/DC converter at the time of engine startup will result in the termination of power supply from the running-purpose battery to the subsidiary battery in accordance with the control apparatus for the hybrid vehicle disclosed in Japanese Patent Laying-Open No. 2003-70103.
In general, the power source such as the subsidiary battery and running-purpose battery has the output power altered according to the charged amount (SOC: State Of Charge) as well as the peripheral environment. Particularly, it is difficult to obviate degradation in the output power from the subsidiary battery due to the continuous driving of a large electric load such as the headlight as well as natural discharge occurring by being left over for a long period of time. At an ECU (Electronic Control Unit) that controls the engine startup upon receiving power supply from the subsidiary battery, there is the possibility of the supplied voltage from the subsidiary battery to the ECU will be lower than the ECU operating voltage.
Since the ECU will be disabled in operation in such an event, safety measures to prohibit activation of the vehicle system (system shutdown) are taken in conventional hybrid vehicles when the voltage of the subsidiary battery becomes lower than a predetermined threshold value. Thus, there was a problem that the engine cannot be started as a result of occurrence of system shutdown at the control apparatus for a hybrid vehicle set forth above.
In view of controlling the drive of a DC/DC converter, Japanese Patent Laying-Open No. 2003-70103 teaches suspension of the operation of the DC/DC converter, and Japanese Patent Laying-Open No. 2003-189401 teaches an operation in two different modes having a different target value for the output voltage.
Specifically, in the hybrid vehicle of Japanese Patent Laying-Open No. 2003-189401, the DC/DC converter activates in a low-voltage mode in which the input electricity of the high-voltage battery is converted into the voltage of 12.0 V corresponding to the level where charging of the control-directed battery of 12 V is not possible, in the case where the power generated by the motor generator is low with the engine in an idling state and the temperature of the high-voltage battery is lower than the defined lower limit temperature. Then, following activation of the DC/DC converter, the generated power is gradually increased at a rate that does not affect the idling rotation of the engine. When the generated amount of power by the motor generator is sufficiently ensured, the operation mode of the DC/DC converter is switched from the low-voltage mode to a high-voltage mode in which the input electricity of the high-voltage battery is converted into the voltage of 14.5 V corresponding to the level where charging of the control-directed battery of 12 V is possible.
Thus, the power consumed by the DC/DC converter output is compensated for by the power generated at the motor generator while ensuring stable idling rotation of the engine. As a result, the discharge from the high-voltage battery at the time of DC/DC converter activation can be suppressed to prevent the temporary voltage drop at the high-voltage battery.
However, when the DC/DC converter is operated in a low-voltage mode in the hybrid vehicle disclosed in Japanese Patent Laying-Open No. 2003-189401, the power consumed by the control computer, accessory, and subsidiary battery connected to the DC/DC converter will be restricted.
In order to ensure the running performance and drivability of the vehicle when the engine is started and attains an idling state, proper operation of all control computers and accessories is required. In the hybrid vehicle of Japanese Patent Laying-Open No. 2003-189401, all the control computers and accessories cannot operate properly when the DC/DC converter is in a low-voltage mode since power cannot be supplied stably to the control computers and accessories. There is a possibility of system shutdown.
In view of the foregoing, an object of the present invention is to provide a power supply control apparatus and method for a hybrid vehicle that can ensure engine startup while avoiding occurrence of system shutdown even in a low temperature environment.